1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, which employs such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
FIG. 1 is a schematic view of one example of fixing device 220.
As shown in FIG. 1, the fixing device 220 includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt 204 entrained around multiple support rollers 202 and 203, paired with a pressure roller 205 that presses against the outer surface of the fuser belt 204 to form a fixing nip N therebetween. One of the belt support rollers is equipped with an internal heater 201, which heats the length of the fuser belt 204 through contact with the internally heated roller 202. As the rotary fixing members 204 and 205 rotate together, a recording sheet S is conveyed through the fixing nip N, at which a toner image on the incoming sheet S is fixed in place with heat from the fuser belt 204 and pressure from the pressure roller 205.
Although advantaged over a configuration that employs a conventional fuser roller instead of a fuser belt, the fixing device 220 described above involves a substantial warm-up time to heat the fixing nip to a temperature sufficient for fusing toner and first-print time to complete an initial print job upon activation. Prolonged warm-up time and first-print time required with the multi-roller belt fuser assembly limits application of the fixing device 220 to relatively slow imaging systems.
FIG. 2 is a schematic view of another example of fixing device 320.
As shown in FIG. 2, the fixing device 320 includes a film-based fuser assembly that employs a fuser belt 304 formed of thin heat-resistant film cylindrically looped around a stationary, ceramic heater 301, which is paired with a pressure roller 305 that presses against the stationary heater 301 through the fuser belt 304 to form a fixing nip N therebetween. As the pressure roller 305 rotates to in turn rotate the fuser belt 304, a recording sheet S is advanced into the fixing nip N, at which the stationary heater 301 heats the incoming sheet S via the fuser belt 304, so that a toner image is fixed in place with heat from the stationary heater 301 and pressure from the pressure roller 305.
Compared to the belt-based fuser assembly, the film-based fuser assembly is superior in terms of processing speed and thermal efficiency. Owing to the thin heat-resistant film which exhibits a relatively low heat capacity, the film-based fuser assembly can be swiftly heated, and therefore eliminates the need for keeping the heater in a sufficiently heated state when idle, resulting in a shorter warm-up time and smaller amounts of energy wasted during standby, as well as a relatively compact size of the fixing device. The film-based fixing device, thus overcoming the limitation of the belt-based fixing device, finds applications in high-speed, on-demand compact printers that can promptly execute a print job upon startup with significantly low energy consumption.
Although generally successful for its intended purpose, the fixing device employing a film-based fuser assembly also has drawbacks. One drawback is its vulnerability to wear, where the heat-resistant film has is repeatedly brought into frictional contact with the stationary ceramic heater. The frictionally contacting surfaces of the film and the heater readily chafe and abrade each other, which, after a long period of operation, results in increased frictional resistance at the heater/film interface, leading to disturbed rotation of the fuser belt, or increased torque required to drive the pressure roller. If not corrected, such defects can eventually cause failures, such as displacement of a printed image caused by a recording sheet slipping through the fixing nip, and damage to a gear train driving the rotary fixing members due to increased stress during rotation.
Another drawback is the difficulty in maintaining a uniform processing temperature throughout the fixing nip. The problem arises where the fuser film, which is once locally heated at the fixing nip by the heater, gradually loses heat as it travels downstream from the fixing nip, so as to cause a discrepancy in temperature between immediately downstream from the fixing nip (where the fuser belt is hottest) and immediately upstream from the fixing nip (where the fuser belt is coldest). Such thermal instability adversely affects fusing performance of the fixing device, particularly in high-speed applications where the rotational fixing member tends to dissipate higher amounts of heat during rotation at a high processing speed.
Vulnerability to wear of a film-based fuser assembly has been addressed by another, improved fixing device that uses a lubricant, such as a low-friction sheet of fiberglass impregnated with polytetrafluoroethylene (PTFE), to lubricate between adjoining surfaces of a stationary pressure pad and a rotatable fixing belt. In this fixing device, the fixing belt is looped for rotation around the stationary pressure pad, while held in contact with an internally heated, rotatable fuser roller that has an elastically deformable outer surface. The pressure pad is spring-loaded to press against the fuser roller through the fixing belt, which establishes a relatively large fixing nip therebetween as the fuser roller elastically deforms under pressure.
According to this arrangement, provision of the lubricant sheet prevents abrasion and chafing at the interface of the stationary and rotatable fixing members, as well as concomitant defects and failures of the fixing device. Moreover, the relatively large fixing nip translates into increased efficiency in heating a recording sheet by conduction from the fuser roller, which allows for designing a compact fixing device with reduced energy consumption.
However, even this improved method does not address the thermal instability caused by locally heating the fixing belt at the fixing nip. Further, this method involves a fixing roller that exhibits a higher heat capacity than that of a fixing belt or film, and therefore requires more time to heat the fixing member to a desired processing temperature during warm-up than would be otherwise required. Hence, although designed to provide an increased thermal efficiency through use of an elastically deformable fuser roller, the method fail to provide satisfactory fixing performance for high-speed, on-demand applications.
To cope with the problems of the fixing device using a cylindrically looped, rotatable fixing belt, several methods have been proposed.
For example, one such method proposes a fuser assembly that employs a stationary, thermal belt holder or heat pipe including a thin-walled, hollow cylindrical tubular body of thermally conductive material or metal. A fuser belt is entrained around the belt holder while heated by a resistive heater such as a ceramic heater disposed in the hollow interior of the belt holder. A coating of lubricant may be deposited on an outer circumferential surface of the belt holder to allow smooth movement of the belt sliding against the belt holder.
According to this method, the thermal belt holder can swiftly conduct heat to the fuser belt, while guiding substantially the entire length of the belt along the outer circumference thereof. Compared to a stationary heater or heated roller that locally heats the fuser belt or film solely at the fixing nip, using the thermally conductive belt holder allows for heating the fuser belt swiftly and uniformly, resulting in shorter warm-up times which meet high-speed, on-demand applications.
In a sophisticated arrangement, the belt holder may be used in conjunction with a contact, fuser pad accommodated in the belt holder inside the loop of the fuser belt to support pressure from the pressure member to establish a fixing nip, as well as a reinforcing member that supports the fuser pad under pressure from the pressure member. Provision of the fuser pad and the reinforcing member allows for stable operation of the fixing device without variations in shape, dimensions, and/or strength of the fixing nip, which would occur where the belt holder itself were subjected to nip pressure, causing deformation and displacement of the thin-walled tubular body.